FIGS. 1A and 1B illustrate the configuration described in Japanese Patent Application Laid Open No. 2015-219273 as a related art example of this type of optical module. In this example, the optical module includes a first optical block 11 and a second optical block 12. The first optical block 11 is fixed on a substrate 14 on which one or more optical elements 13 are mounted. The second optical block 12 holding one or more optical fibers 15 is mounted on the first optical block 11. The number of the optical fibers 15 is generally equal to the number of the optical elements 13, and the number of the optical elements 13 is usually greater than or equal to 2.
In a state that the second optical block 12 and the first optical block 11 are interlocked, the optical elements 13 are positioned in a space 11a which is formed on a bottom surface of the first optical block 11. Lenses 11b, the number of which is equal to the number of the optical elements 13, are formed on the first optical block 11. One of the lenses 11b is positioned just above a corresponding one of the optical elements 13 in the state that the second optical block 12 and the first optical block 11 are interlocked.
The second optical block 12 includes four pieces of spring pieces 12a. The four spring pieces 12a are formed in a manner to be integrated with a main body part of the second optical block 12. A claw part 12b is formed on an end of each of the four spring pieces 12a. The second optical block 12 is pushed to the top of the first optical block 11 to fit with the first optical block 11. At this time, the claw parts 12b are hooked on an upper end of a step part 11c formed on the first optical block 11 and thus, the second optical block 12 is fixed on the first optical block 11.
A convex part 12c is formed on a bottom surface of the second optical block 12. The convex part 12c is fitted on a concave part 11d formed on an upper surface of the first optical block 11. Lenses 12d, the number of which is equal to the number of the optical elements 13, are formed on the convex part 12c. In the state that the second optical block 12 and the first optical block 11 are interlocked, an optical axis of each of the lenses 11b is accorded with an optical axis of a corresponding one of the lenses 12d. A reflection surface 12e is formed on an upper surface of the second optical block 12. The reflection surface 12e is positioned just above the lenses 12d. 
The optical elements 13 are light emitting elements or light receiving elements, for example. In the case where the optical elements 13 are light emitting elements, each of the lenses 11b converts light from a corresponding one of the optical elements 13 into parallel light. Each of the lenses 12d collects the parallel light from a corresponding one of the lenses 11b. The reflection surface 12e converts travelling directions of all the light from the lenses 12d by 90°. Each of all the light from the reflection surface 12e is incident on a corresponding one of the optical fibers 15. The optical fibers 15 are held by the second optical block 12. The extending directions of the optical fibers 15 are parallel to the substrate 14.
Thus, the optical module illustrated in FIGS. 1A and 1B optically connects the optical elements 13 to the optical fibers 15. A material of the first optical block 11 is resin which transmits light. The first optical block 11 is formed by resin molding. A material of the second optical block 12 is resin which transmits light. The second optical block 12 is formed by resin molding.
Accordingly, the spring pieces 12a are formed also by resin molding. In order to easily mold the spring pieces 12a and obtain a required spring force (holding force), the width of the spring pieces 12a (that is, a dimension in a direction parallel to a plate surface of the substrate 14) has to be set large. As a result, the optical module is increased in size.